1. Field of the Invention
This invention relates to a shutter device for a camera and more particularly to a motor-driven shutter.
2. Description of the Related Art
Programmed shutters of the kind having sectors which form a lens aperture, a stepping motor which rotates stepwise forward or backward according to the phase sequence of driving pulses time-serially supplied, a transmission gear train which transmits the rotation of the stepping motor to the sectors and an arrangement for controlling the opening and closing of the sectors through the rotation of the stepping motor, have been known from, for example, Japanese Utility Model Publication No. HEI 4-47697, Japanese Patent Application Laid-Open No. HEI 3-184029, Japanese Utility Model Laid-Open No. SHO 62-89624, Japanese Patent Application Laid-Open No. SHO 57-150829 (corresponding to U.S. Pat. No. 4,325,614), etc.
Generally, however, the stepping motors such as the motors employed in the prior art examples mentioned above do not permit to increase their rotation speed at the start of driving and at the time of change-over from forward rotation to backward rotation. The speed of the sectors thus cannot be increased at such a time. This shortcoming of the stepping motor has necessitated use of only a half-open area of the aperture when it is necessary to obtain a high shutter speed for a high luminance field.
A shutter permitting use of only the half-open area for a high luminance field presents problems in that the intrinsic power of a photo-taking lens cannot be fully used and that, in the event of fill-in flash photography, the exposure interlocking (synchronizing) distance of the flash device becomes nearer.
To solve these problems, it has been contrived to arrange a shutter device to include a first shutter which is normally closed and a second shutter which is normally open, to open the first shutter and close the second shutter by energizing them in such a way as to cause the second shutter to cover an aperture formed by the first shutter and to obtain a required amount of exposure by electrically controlling the points of time at which the first and second shutters respectively begin to operate.
However, in order to obtain a high shutter speed even for the full open area of the aperture, the shutter device of this kind necessitates increasing also the shutter operating speed. Unevenness in length of time required before the actual start of operation of the first and second shutters after receipt of an operation start signal results in an error in the amount of exposure. Particularly, since the conventional shutter device uses an electromagnetic device as a drive source for driving the shutters, the unevenness in operation start time is great and further increases accordingly as the shutter speed increases to give a greater error in amount of exposure for a high shutter speed.